Miata Mailing List: January 2000, Message #153

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To all:

I am looking for some help or charity regarding an
exhaust rattle on my '91 A-pkg with 102k miles:

To start, here is my current exhaust setup:
Brospeed header, hollowed out factory cat, and Thermal
Research cat back system.

Here is my previous exhaust setup:
Brospeed  header, JR hi-flow cat, and Ansa muffler.

Here is my "previous to previous" setup:
Stock header, JR hi-flow cat, Ansa muffler.

Before that setup:
Stock header, stock cat, Ansa muffler.

And, initially, the stock setup.

During this whole chain of exhaust events, I have had
a rattle, initially at 3500-4500 rpms.  Now, it is at
2000-2500 rpms, disappears, and reappears at 3800-4500
rpms.  Of course, with the hollowed out factory cat,
it seems louder than before, but just as obnoxious.  I
have cleaned out any debris from the heat shields and
left off the shield from the header.

To my question(s):
Is it my cat that is rattling?  Is it possible to have
both a bad stock cat and a bad JR hi-flow cat?  The
problem is only present when the driveline is under
load.  I can be in neutral, rev the car and get a very
faint rattle, but nothing like the racket I get when I
am under way.  Any ideas?
If anybody has an available stock cat they know
doesn't rattle, would you be willing to donate it to
me in the name of scientific discovery =) ?  

Please let me know where else I may need to look for
the cause of the rattle.  Maybe I should get a high
flow dog instead ;-).

TIA,
Jay

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